Sintered refractory material



abandoned.

,- "contains tion i's iso vague that i stiti stituent.

UnitedA StatesPatct iCe l `r2,829,427 f siNrEREn `REFRAoroRr MATERIAL Skon Tacvorin: and MarcellLevecque, Paris, and Ren Leclerc, Lai `Roche,I Toulouse-Pouvourvlle, Erancey assignors to Olice National dEtudes et-de Recherches Aeroriantiques,A Chatillonysous- Bagneum France n iNoDrawm'g-ill Application April-14,1953 i fuseriamasfiasos 1 A pnrityf, lippliain Trigue? oftbgr 1s, 194s 1s claims. (0.2941325) n abandoned, which is a continuation-impart of applica- This invention relates to sintered refractory materials. i

` i 3 The use of sintered alumina has previously been suggested for cutting tools andattempts have been made i to improve` the mechanical characteristics of sintered alui minafb addingfthei'eto small amounts of oxides oflchrol l mim, iron,` vanadium, onman'ganese, andso ffoxt-h,A HoW- ever, while thehardness of the products th'u s obtained"isf equivalent l to that of plates'oi metallic rcarbides, their B'sids; same ,writers hat@ and mentioned 'that "in as a listwit-lranother taken tofbtin arefractory`,co1npositioi1;` But, this prescripr' technieianjeoula @my cuentan-g dredsit notthousa some of 'these compos ons.' t

It is an `objeot t"'th1sf i`nven`tionfto provide fa' :nat which may j befused wherever high resistance ,to creep at"7 high temperature 0 contemplates providing the bond between these substances by the use of'one or more metal sesquioxides l application is `a continuation-impart ofapplication Serial No. 194,737, filed November 8, 1950 and now 2,829,427. Patented lApr.4 8, 1958 The Iinventionsprovides, to be advantageously -assof ciated with alumina, a metal of the sixth group of the Mende1yeev,ClasSication or an alloyof such a metal with other metals of the.V same groups or of the iron group which, ias isxknowmincludes iron, nickel and cobalt.

As the metallic constituentto be associated'with magnesia, the invention provides :a metal of the iron group or an alloy containing a metal of this group.

In the .case of alumina and. chromium,- the invention with alumina a such as Cr203 and Mn203.

The average expansion coefficients of the ceramic con'-V I stituentandof the metal constituent should be suicientlyl close lto each other that, when subjected to thermal shocks,

4-in use, the strains which developin the material cannot break' the` bond provided by vthe'complementary oxide'. Practically, the average expansion coefficients of the metal constituent on one hand, and of the ceramic constituent` 1 on the other hand, should not differ from more than 20% i i This valueis liable tovary with the granulometry of the constituents` and is at the highestwhenthe granulometry is the nest;v i

`The mechanical-'characteristics, resistance to thermal shockvand tti-creep,` vary with the concentrations off the various constituents and may be adapted to any individual instancefof: use. l

However-the; proportions of the main constituents, i. re; refractory oxideaandcmetal, have aV considerable significance; some of the properties of Ythe material and particularly those whichdisclose resistance tothermal ab'ut 90o` C., it hasbeen fimd lriot 'to' retain as me-f 35 chanical qualities with time. also" given lists Yoli elements In4 this connection, the invention contemplates'the;

plication` of lsai d Iinaterial to ,the constructionigzafjhtat engineeleml'ent especiall" rbinebladesandpr pulsion the-material cmprissfait and@ nistal .con

ts'iofwhia at'metal 'oxide o I i U u ww m'. ent; t e average ex ansioncoe c equal or close to each other, andalso complementary oxide, synrystallizable with the refraci, constituent. n f @It is--toA be understood that wfmetal-constituent" means 60 all thevmetal-elementsforming part of shocks show rapid u class of' materials may be'deiined which,`although Ahaving common properties, have each their own particularity.

.The'materials vcomprising alumina, chromium `oxide and chromium, this latter alone, or as alloy with other metalsas 'mentioned above,"1are of primary importance.`

Sucliimaterials belong tofseveral classes.

, .These Vmaterials'have infcommon various properties,

especially refractory character, resistance towards; oxida-V gtiong the possibility oa-good sintering, no creepingvat* 1,000iiC., an 4expansion 'coefficient substantially constantand similar mechanic properties at low tempera-' l ture.` i i VOther properties show a linear variation with thel'relaL ,tive -proportion of the vtwo main i constituents: These properties are,"for'example, density, hardness, resistance` tocreeping Vat high temperature.

y On theother: hand, some other properties sho'w sudden variations for given values of the percentage inthe metal constituent, and this defines a numberof classes differthe `material, tliese'l ,as

tary oxide has, said zoxidevi venti'onfcomprises-in itsS-scope ompositionsiiinrwhichthe 70 thoria.

ing-` from each other in said properties. Among these latter, are;Y thermal conductivity, resistance to thermalv sho'ck. i l

`.1st Class.'-The pecelntage in weight of the metal con;

"ilstituent, which is totally :of Paftauy chromium is con" prised'betwe'en 2 tolS and 25 lto 30 of thetotal weight ,2d Class- The;limits are: 25 to 30% and 40 -to 50%.,

dfcJqsaT-The umits are: 40 to 50% and 7o to 80%.

The proportionsfxing the transition are dependent,"

inside these limits, of the granulometry of the metal constituent.

"In"'acl1e1ass, anumber of families may be separated, according to the composition of the metal constituent.

QFor example, the materials of the second class maybe e secondfamily, the materials of which coniprisef'lY divided into three families, as hereafter:

"A rst familyin which the materials comprise, as metalH constituent,.chromium only, and which are particularly;

resistant to corrosion;

isomorphous l variations for some proportions, and

as metal constituent, chromium and a metal of the sixth group, and which are highly refractory;

The third family, the materials of which comprise, as metal constituent, chromium, a metal of the sixth group and one or several metals of the iron Vgroup and which are obtainable at low price.

For the construction of heat engine elements, the materials of the third class are preferred as they show simultaneously, beside the qualities which are common to all classes, a high resistance to thermal shock associated with a good refractory character.

For the production of heating rods, the materials of the first class are preferred as they show simultaneously, beside the qualities common to all the classes, an electric resistance high enough to require a not too high heating current.

For the production of propulsion jets nozzles for missiles to be used only once, for example, the materials -of the second class, which have a thermal resistance of short duration, are very well adapted.

To produce a material according to the invention, the powder-metallurgical sintering techniques are advantageously employed. The constituents in the form of fine powders are intimately mixed, and the mixture may -additionally contain a small amount of one or more mineralizing agents such as magnesia.

Forming is effected by pressing, casting or extrusion depending on the nature and concentrations of the respective constituents and the shape of the articles to be produced.

Sintering is accomplished at a temperature from 1,500 to 1,900 C., preferably in a controlled furnace atmosphere which depends on the character of the substances present in the mixture.

Thus, the additional oxide may be produced in situ through moderate oxidisation of the metal or alloy.

In some cases, ,it is possible to cause formation of covalent binary compounds adapted to promotethe metalto-oxide bond and improve the characteristics of the material.

In order to obtain a sintered material comprising alumina, chromium and chromium oxide, the invention provides preferably to heat the material to be sintered at first in a hydrogen atmosphere up to a temperature of about 1,200 C., then in a nitrogen atmosphere. A rapid sintering is thus obtained at a relatively low temperature and without swelling of the material, which would occur if the sintering were `to be carried out permanently in a nitrogen atmosphere.

The speeding up of the sintering may be explained by the fact that at a temperature of about 1,600 C. a eutectic chromium nitride-chromium is formed, due to the presence of nitrogen and which wets the particles to be sintered, the sintering taking place in liquid phase.

The same advantageous results are obtainable, Without swelling being observed, by replacing in the initial mixture the chromium powder with chromium nitride and carrying out the sintering in a nitrogen atmosphere, which provides, besides, at high temperatures, the` same equilib rium chromium-chromium nitride.

Taking into account the contiguous phenomena, the proportions of the metals in the sintered material, and which `are given hereafter in the examples, comprise not only the quantity which is to be found in the metal condition proper, but also to be found with nitrogen.

Example I The following constituents in the indicated proportions are used in the form of a powder of 'such fineness that more than 50% of the constituents comprise particles less than 5 microns in size, and the major part of the remainder of the particles being less than l0() microns in size. The powdered constituents are intimately mixed, after having desirably added thereto any of the lubricants generally used n the powder-metallurgy art, such as glycerine, camphor, parafiin, glycol, etc.

The resulting mixture is placed in a mold and compressed therein to a pressure approximating 2 tons per sq. cm. in au ordinary press.

The pressed article is placed in a furnace at a gradually increased temperature, in a controlled atmosphere: a hydrogen atmosphere up to 1,000 to 1,400 C., then a nitrogengatmosphere up to 1,700 to 1,750 C. At the end of this operation, a material is obtained which possesses the following characteristics: Tensile strength: 25 kg./sq. mm.; excellent erosion resistance, and resistv ance to oxidization and creep at high temperature.

Example 2 The following constituents in the following proportions Percent A1203 50 CI203 Cr 49.2

are reduced to a fine powder such that more than 50% of the constituents comprise particles less than 5 microns in size while the major part of the remaining particles is less than 100 microns in size. p

They are mixed with water to form a barbotine or slip of suitable viscosity in view of the yshaped articles to be produced. This slip is stabilized by the addition thereto of an acid, suitably hydrochloric acid, in a proportion of a few thousands of one part of acid for one part mixture.

This paste or slip is cast in a pla-ster mold of plaster,

baked clay or similar absorbent material. After-a suitl able wall thickness has been obtained, the poured off.

If it is desiredto produce acomposite article, then this first slip or barbotine is replaced by a further barbotine of different composition, containing for example a refractory oxide, such as pure alumina. The substance or substances contained in this second barbotine settle over the initial article already formed. The particles size of the second barbotine is so selected that both layers will have an identical degree of shrinkage during the sintering operation. The part is removed from the mold and very slowly dried in air first at room temperature, then at C.

The dried article is placed in a furnace in a nitrogen atmosphere and carried to 1,650 C., which is the -sintering temperature.

After the sintering has been completed, the resulting articleisv non-permeable, possesses a breaking load of 20 excess slip is to 30 kg./sq. mm., with excellent resistance to high temperature creep, oxidization and sharp temperature variations or thermal impact, thus making it particularly suitable for use as a jet-nozzle.

Example 3 The following substances inthe following proportions:

Percent 56 4 30 Mo l0 are used in the form of a ne powder, the granulometrical analysis similar to thatdei'ined abov The constituents are mixed with a small amount of an organic solvent, e. g., ether, alcohol, benzene and a lubricant such as stearic acid or a stearate.

The paste may be subjected to a vacuum to remove any bubbles formed in it. The desired articles are then shaped by extrusion.

The extruded larticle is slowly dried in air, first atfroom temperature, then: at 60 C. The dried article is placed in `a furnace in which a controlled atmosphere is maintained as follows: hydrogen atmosphere up to .1,000-

1,400 then nitrogen atmosphere up to` 1,650 1,7`5V0"` C.

The shaping. is "accomplishedl-by! pressing, coating or extrusion. It is thus possible `to obtain a rod of such an electric conductivity that it may be used uas heating resistor. This rod may optionally be coated with a protective coating, of e. g. alumina. Such a 'rod is capable of i withstanding temperatures `up to 1,600: C. vfor long periods oftime, even in an oxidizingatmosphere.

They are used in the form of a fine powder, the granu- They are thoroughlymixed after advantageous admix- "ture of a lubricant, as hereabove mentioned.

` sq. mm. andan excellent resistance to thermal shocks.

These constituents are used in theform of tine powders which, for alumina and chromium, have a granulometrical analysis similar to that mentioned in Example 1; as far as .the mineralizing agent is concerned, magnesia in the present case, it is introduced as an extremely fine powder advantageously obtained through` calcination of the precipitated hydrocarbonate.

The whole is lshaped by pressure, as in Example and is placed in a furnace at a` gradually increasing temperature in controlled atmosphere; an hydrogen atmosphere is maintained up to `1,000--l,400, then a nitrogen atmos- 1 Despite the use of the alternated atmosphere .which `avoids the swelling of the mass during the'sintering, a slight swelling is observed in this case from 1,000 and reaches its maximum at about 1,300". This is due to the combination of alumina with magnesia which gives rise `to an armature whch is consolidated when the metal constituent melts.

In the Examples 2-4-5-6, the chromium oxide may be obtained in situ, before the sintering by partial oxidation of the chromium below 1,200, for example in admitting into the furnace atmosphere a quantity of oxygen or steam calculated according to the proportion of chromium oxide to obtain. l

H. lometricalanalyss being similar to that mentioned in "3 Example 1.

Example 6 The constituents and their proportions are as follows:

` Percent `A1203 21 MgO 7 Cr 71.5 CI'2O3 .5

The following constituentsare` used in the indicated i proportions:` `l.12`o3 f L Under 70% vCr t Over 30%. `Cr203 *Traces What WeA claimis: I

l. A method for producing a shapedarticle of `sinterelil refractory material comprising the steps of shaping Aa mix-` ture in finely divided form `of at least 20% alumina, i

2-80% chromium and .3-4% chromium oxide, and heat-` ing` the shaped mixture first ina hydrogenfatmosphere up to about 1,200 C. then in anitrogen atmosphere` to the sintering temperature.

2. A sintered refractory materialhaving suhstarrtiallytf,

the following composition:

3. A sintered refractory material having substantially t Vthe following composition:

. Percent A1203 y i CI'203 i 0.8

4. A sintered refractory material having substantially the following composition: M tt tt 'Percent A1203 01.203 4 Cr 30 Mo 10 5. A sintered refractory material having substantially the following composition:

. Percent A1203 30 Cr 69.5 CI'ZOa 0.5

the following composition:

A refractory material consisting essentially of, in intimately bonded relationship resulting from sintering: alumina 1n an amount at least equal to 20% by weight, 280% chromium and chromium oxide in an amount comprised between 0.3% and 4%. A refractory material consisting essentially of, in intimately bonded relationship resulting from sintering: alumma 1n an amount at least equal to 20% by weight, 2-80% chromium and another metal selected from the group lconslsting of molybdenum and tungsten, the average coefficlent of expansion of the metallic constituent comprislng chromium and said other metal dilering by no more than 20% from that of alumina, and chromium oxide 1n an amount comprised between 0.3% and 4%. Y `9. A refractory material consisting essentially of, in intimately bonded `relationship resulting from sintering: alumina in an amount at least equal to 20% by weight, 2-80% chromium and an iron-group metal, the average coefcient of expansion of the metallic constituent comprising chromium and said other metal differing by no more than 20% from that of alumina, and chromium oxide in an amount comprised between 0.3% Yand 4%.

l0. A refractory material consisting essentially of, in intimately bonded relationship resulting from sintering: alumina in an amount at least equal to 30% by weight, 2-80% chromium and an alloy of a metal selected from the group consisting of molybdenum and tungsten 'with an iron-group metal, the metallic constituent comprising chromium and the said alloy having an average coefficient `of expansion differing by less than 20% from that of '7 alumina, and chromium oxide in an amount comprised between.0.3% and 4%.

l1. 'A refractory material consisting essentially of, in intimately bonded relationship resulting from sinteriug: alumina, chromium inan amount comprised vbetween 25 and 50% 4by Weight, and chromium oxide in an amount comprised between 0.3% and 4%.

12. lA` refractory material consisting essentially of, in intimately bonded `relationship resulting from sintering: alumina, a metallic constituent comprising7 chromium, a metal selected from the group consisting of molybdenum and tungsten and an iron-group metal, the average coelicient of expansion of the metallic constituent differing'from that of alumina by less than 20%, the proportion of the metallic constituent of the total weight being comprised between`25 and 50%, and chromium oxide in an amount comprised between 0.3% and 4%,

13. A refractory material consisting essentially of, in intimately bonded relationship resulting from sintering: alumina, chromium in an amount comprised between 40 and 80% by weight, and chromium oxide in an amount comprised between 0.3% and 4%.

14. A refractory material consisting essentially of, in intimately-bonded relationship resulting from sintering: alumina, a metallic constituent comprising chromium, a metal selected from the group consisting of molybdenum and tungsten and an iron-group metal, the average coecient of expansion of the metallic constituent differing by less than 20% from that of alumina, the metallic constituent being in an amount comprised between 40 and 80% by weight, and chromium oxide in an amount comprised between 0.3% and 4%.

15. A refractory material consisting essentially of, in intimately bonded relationshipresulting from sintering: alumina in an amount .greater than 20% by weight, 2-"80% of a metallic constituent comprising chromium and having a coeicient of expansion differing by less than 20%l from that of alumina, chromium oxide in an amount vcomprised between 0.3% and 4%, and magnesia as a mineralizing agent.

References Cited in the le of this patent UNITED STATES PATENTS 

7. A REFRACTORY MATERIAL CONSISTING ESSENTIALLY OF, IN INTIMATELY BONDED RELATIONSHIP RESULTING FROM SINTERING: ALUMINA IN AN AMOUNT AT LEAST EQUAL TO 20% BY WEIGHT, 2-80% CHROMIUM AND CHROMIUM OXIDE IN AN AMOUNT COMPRISED BETWEEN 0.3% AND 4%. 